Momentum Explained

Trends tend to keep going longer than fundamentals alone can justify. Momentum is one of the most persistent, well-documented patterns in markets, and one of the hardest to explain cleanly.

Trends that outlast the news

Momentum describes the tendency of assets that have recently performed well to keep performing well over the following months, and assets that have recently performed poorly to keep underperforming, beyond what a simple efficient-market view would predict. If prices instantly and fully reflected all available information, past returns should carry no useful information about future returns, momentum's persistence across decades of data, across asset classes and geographies, is one of the more durable challenges to that simple efficiency assumption.

This isn't the same as saying what goes up must keep going up forever. Momentum describes a statistical tendency over specific horizons, typically several months to about a year, not a permanent state, and it coexists, somewhat uncomfortably, with mean reversion showing up over different horizons and in different variables.

Behavioral explanations

The leading explanations for momentum are behavioral rather than purely mechanical. Underreaction is one piece: investors, and analysts in particular, tend to update their views on new information more slowly and incrementally than a perfectly rational actor would, meaning good or bad news gets fully absorbed into price gradually over subsequent months rather than instantly, producing a trend as the market catches up to information it initially underweighted.

Herding is the other major piece: as a trend becomes visible, it attracts attention and capital independent of the original fundamental driver, investors who see an asset performing well assume other participants know something they don't, and buy in response to the price action itself rather than new information. This can eventually overshoot into the kind of positive feedback loop that pushes momentum past what any reasonable fundamental reassessment would support.

Why momentum and mean reversion can both be true

The apparent contradiction resolves once time horizon and the specific variable in question are accounted for. Short and medium-term price momentum, assets trending for weeks to months, driven by underreaction and herding, can coexist with long-term mean reversion, valuations and extreme sentiment eventually normalizing over years, because they're describing different mechanisms operating on different clocks. A stock can exhibit genuine positive momentum for the next two quarters while also being a strong mean-reversion candidate on a five-year valuation view.

Regime also matters: momentum tends to work best in trending, directional markets with a clear macro or narrative driver, while mean reversion tends to dominate in range-bound, choppy environments where no single trend has enough conviction behind it to sustain itself.

The practical risk

Momentum's central danger is timing the turn. Because momentum is, by construction, a trend that has already been running for a while, buying into strong momentum means buying into a move that's already partially exhausted its easiest gains, and the same herding dynamics that build momentum can reverse sharply and without much warning once the underlying driver fades or a crowded position starts to unwind.